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United States Patent |
5,057,808
|
Dhyanchand
|
October 15, 1991
|
Transformer with voltage balancing tertiary winding
Abstract
A summing transformer includes first and second sets of primary windings,
first and second sets of secondary windings inductively linked with the
first and second sets of primary windings and a set of tertiary windings
inductively linked with one of the first and second sets of secondary
windings wherein the tertiary windigs are connected together and permit
zero sequence currents to flow during a period of unbalanced load currents
in the sets of secondary windings whereby the output voltages produced
thereby are maintained in a substantially balanced condition during such
period.
Inventors:
|
Dhyanchand; P. John (Rockford, IL)
|
Assignee:
|
Sundstrand Corporation (Rockford, IL)
|
Appl. No.:
|
457944 |
Filed:
|
December 27, 1989 |
Current U.S. Class: |
336/12; 363/43 |
Intern'l Class: |
H01F 033/00 |
Field of Search: |
363/43,72
323/361
336/5,10,12
|
References Cited
U.S. Patent Documents
963132 | Jul., 1910 | Frank | 336/12.
|
1173094 | Feb., 1916 | Blume | 336/12.
|
1812949 | Jul., 1931 | Halperin et al. | 336/12.
|
2779926 | Jan., 1957 | Johnson et al. | 336/15.
|
3246267 | Apr., 1966 | Wesolowski et al. | 336/5.
|
3611224 | Oct., 1971 | Becker | 336/5.
|
3775662 | Nov., 1973 | Compoly et al. | 363/43.
|
Primary Examiner: Kozma; Thomas J.
Attorney, Agent or Firm: Marshall, O'Toole, Gerstein, Murray & Bicknell
Claims
I claim:
1. A summing transformer, comprising:
a first set of primary windings receiving a first set of polyphase
currents;
a second set of primary windings receiving a second set of polyphase
currents;
a first set of N secondary windings inductively linked with the first set
of primary windings and developing a first set of secondary polyphase
voltages;
a second set of N secondary windings inductively linked with the second set
of primary windings and developing a second set of secondary polyphase
voltages, each winding of the second set of secondary windings being
coupled in series with a corresponding winding of the first set of
secondary winding to form N series-connected pairs of secondary windings
across which N output voltages are produced; and
a set of N tertiary windings inductively linked with one of the first and
second sets of N secondary windings, the tertiary windings being connected
together in a delta configuration and permitting zero sequence currents to
flow during a period of unbalanced load currents in the N series-connected
pairs of secondary windings whereby the output voltages are maintained in
a substantially balanced condition during such period.
2. The summing transformer of claim 1, wherein the windings of the first
set of primary windings are connected in a wye configuration and the
windings of the second set of primary windings are connected in a delta
configuration.
3. The summing transformer of claim 2, wherein the set of N tertiary
windings are inductively linked with the first set of N secondary
windings.
4. The summing transformer of claim 1, further including third and fourth
sets of primary windings which receive third and fourth sets of polyphase
currents, respectively, and third and fourth sets of secondary windings
inductively linked with the third and fourth sets of primary windings,
respectively, wherein each winding of the fourth set of N secondary
windings is coupled in series with a corresponding winding of the third
set of N secondary windings and one of the N series-connected pairs of
secondary windings.
5. The summing transformer of claim 4, including a further set of N
tertiary windings inductively linked with one of the third and fourth sets
of N secondary windings, the further set of N tertiary windings being
connected together in a delta configuration.
6. The summing transformer of claim 5, wherein the further set of N
tertiary windings are inductively linked with the third set of N secondary
windings.
7. The summing transformer of claim 1, further including third through
sixth sets of primary windings which receive third through sixth sets of
polyphase currents, respectively, and third through sixth sets of N
secondary windings inductively linked with the third through sixth sets of
primary windings, respectively, wherein corresponding windings of the
third through sixth sets of N secondary windings are connected in series
with corresponding series-connected pairs of secondary windings.
8. The summing transformer of claim 7, including two further sets of N
tertiary windings inductively linked with two of the third through sixth
sets of N secondary windings, each further set of N tertiary windings
being connected together in a delta configuration.
9. The summing transformer of claim 8, wherein the two further sets of N
tertiary windings are inductively linked with the third and fifth sets of
N secondary windings.
Description
TECHNICAL FIELD
The present invention relates generally to transformer construction, and
more particularly to a polyphase transformer for use in a power generating
system.
BACKGROUND ART
Variable speed, constant-frequency power generating systems are often used
to convert variable-speed motive power produced by a prime mover into
constant-frequency AC power. Typically, such systems include a brushless,
synchronous generator which converts the variable-speed motive power into
variable-frequency electrical power, a rectifier which converts the
variable-frequency electrical power into DC power and a polyphase inverter
which converts the DC power into the constant-frequency AC power. The
inverter may be of the stepped-waveform type wherein a series of polyphase
subinverters are interconnected by a summing transformer which in turn
produces polyphase stepped-waveforms each approximating a sinusoidal
output.
Occasionally, an unbalanced condition can arise in loads connected to the
inverter output. This, in turn can cause the output voltage magnitudes of
the summing transformer to become unbalanced or substantially unequal.
Such a condition is undesirable where output voltage magnitudes must be
held within a certain range of a specified level.
A 24-step waveform inverter having a summing transformer which sums the
outputs of four subinverters is disclosed in Compoly et al., U.S. Pat. No.
3,775,662.
A voltage balancing circuit for a polyphase inverter is disclosed in Mehl,
co-pending application Ser. No. 607,811, filed Nov. 1, 1990 entitled
"Voltage Balancing Circuit" and assigned to the assignee of the instant
application. The voltage balancing circuit includes a pair of supplemental
inverters producing supplemental AC power which is combined with the
output power on two of three output phases of the inverter so that all of
the phase voltage magnitudes are maintained within a range of voltage
magnitudes.
SUMMARY OF THE INVENTION
In accordance with the present invention, the outputs of a stepped a
waveform inverter are balanced in a simple and inexpensive fashion.
More particularly, a summing transformer includes first and second sets of
primary windings and first and second sets of N secondary windings
inductively linked with the first and second sets of primary windings,
respectively. Each winding of the second set of N secondary windings is
coupled in series with a corresponding winding of the first set of
secondary windings to form N series-connected pairs or groupings of
secondary windings across which N output voltages are produced. A set of N
tertiary windings are inductively linked with one of the first and second
sets of N secondary windings. The tertiary windings are connected together
in a delta configuration and allow zero sequence current to flow during a
period of unbalanced load currents in the N series-connected pairs of
secondary windings whereby the output voltages are maintained in a
substantially balanced condition during such period.
The transformer of the present invention does not require the use of active
balancing circuits under most operating conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a prime mover in conjunction with a
variable-speed, constant-frequency (VSCF) power conversion system;
FIG. 2 is a block diagram illustrating the VSCF system of FIG. 1 in greater
detail; and
FIGS. 3A and 3B when joined at the dashed lines, together comprise a
simplified schematic diagram of the stepped waveform inverter of FIG. 2
including the summing transformer of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, a prime mover 10, such as an aircraft jet engine,
develops variable-speed motive power which is converted by a VSCF system
12 into constant-frequency AC electrical power on a load bus 14. It should
be noted that various contactors for connecting the VSCF system 12 to the
load bus 14 are not shown for the sake of simplicity.
Referring now to FIG. 2, the VSCF system 12 includes a brushless,
synchronous generator 20 which converts the variable-speed motive power
produced by the prime mover 10 into variable-frequency power. An AC/DC
converter 22 which may be, for example, a rectifier bridge of conventional
design, converts the variable-frequency power into DC power on a DC link
24 comprising first and second DC link conductors 24a, 24b. The conductors
24a, 24b are coupled to a DC/AC converter 26 which may be a three-phase
stepped-waveform inverter which produces three-phase output voltages and a
neutral voltage on a set of output lines 28a-28c and 28n. The output lines
28a-28c and 28n are coupled to an optional filter 30 which is in turn
coupled to the load bus 14. The VSCF system 12 further includes regulation
circuitry (not shown) for regulating the output of the inverter 26. Such
circuitry forms no part of the present invention, and hence will not be
described in greater detail herein.
Referring now to FIG. 3, the stepped-waveform inverter 26 includes first
through sixth subinverters 41-46 each including first through sixth power
switches Q1-Q6, Q7-Q12, Q13-Q18, Q19-Q24, Q25-Q30 and Q31-Q36,
respectively. The power switches may be of any suitable type, such as
conventional bipolar transistors or insulated gate bipolar transistors.
Connected in antiparallel relationship across each power switch Q1-Q36 is
a flyback diode D1-D36, respectively. Phase outputs 50a-50c, 52a-52c,
54a-54c, 56a-56c, 58a-58c and 60a-60c of the subinverters 41-46 are
coupled to first through sixth sets of primary windings 61a-61c through
66a-66c of a summing transformer 68. The windings of the first, third and
fifth sets of primary windings 61a-61c, 63a-63c and 65a-65c are connected
together in a wye configuration whereas the windings of the remaining sets
62a-62c, 64a- 64c and 66a-66c are connected together in a delta
configuration. First through sixth sets of secondary windings 71a-71c
through 76a-76c are inductively linked with the sets of primary windings
61a-61c through 66a-66c, respectively. In known fashion, first through
sixth sets of stepped polyphase currents are developed by the subinverters
41-46 and flow through the first through sixth sets of primary windings
61a-61c through 66a-66c, in turn inducing first through sixth sets of
polyphase voltages in the secondary windings 71a-71c through 76a-76c,
respectively. Corresponding windings of the six sets of secondary windings
are connected together in series to form three series-connected groupings
of windings wherein a phase output voltage V.sub.A, V.sub.B or V.sub.C is
formed across each grouping and is provided on a line 28a, 28b or 28c to
the filter 30. More specifically, the windings 71a-76a are connected in
series between the line 28a and the neutral line 28n. The windings 71b-76b
are coupled between the lines 28b and 28n while the windings 71c-76c are
coupled in series between the windings 28c and 28n. Capacitors C1-C3 are
coupled between the lines 28a-28c, respectively, and the neutral line 28n.
First, second and third sets of tertiary windings 81a-81c, 83a-83c and
85a-85c are inductively linked with the first, third and fifth sets of
secondary windings 71a-71c, 73a-73c and 75a-75, respectively. The tertiary
windings of each set 81a-81c, 83a-83c and 85a-85c are connected together
in a delta configuration. It can be seen from FIG. 3 that the tertiary
windings are inductively linked with summing transformer portions having
primary windings connected in a wye configuration.
During operation of the inverter 26, an unbalanced load condition can arise
wherein the currents flowing in the lines 28a-28c become unequal. Such an
unbalanced load current condition, can, if uncorrected, cause voltages at
one or more primary neutral points PN1-PN3 to vary from a neutral voltage,
in turn causing the line-to-neutral voltage magnitudes produced in the
associated sets of secondary windings 71a-71c, 73a-73c or 75a-75c to
become unequal. These voltage magnitudes can be resolved to positive,
negative and zero sequence voltage components. The closed tertiary
windings present an infinite impedance to the positive and negative
sequence voltage component and present a very low impedance to the zero
sequence voltage component. The tertiary windings, therefore, effectively
short-circuit the zero sequence voltage components which are the only
components that contribute to the unbalanced voltage condition. Thus, the
unbalanced voltage condition is prevented by the tertiary windings.
It should be noted that the summing transformer 68 illustrated in FIG. 3
need not include six sets of primary, secondary and tertiary windings.
Instead, the transformer could comprise two or four sets of primary and
secondary windings coupled to two or four subinverters, respectively. In
such a case, half of the primary windings would be connected in a wye
configuration whereas the remaining half of the primary windings would be
connected in a delta configuration. One or two tertiary windings would
then be inductively linked with the secondary winding(s) that are
inductively linked with the primary windings connected in a wye
configuration.
It can be seen that the summing transformer according to the present
invention is simple in design and does not require the use of active
components for balancing the voltages produced thereby.
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